1. Field of the Invention
The invention relates to radio frequency identification (RF/ID) interrogators and transponders, and more particularly, to an RF/ID interrogator that can recover data from moving RF/ID transponders.
2. Description of Related Art
In the automatic identification industry, the use of RF/ID transponders (also known as RF/ID tags) has grown in prominence as a way to track data regarding an object to which the RF/ID transponder is affixed. An RF/ID transponder generally includes a semiconductor memory in which digital information may be stored, such as an electrically erasable, programmable read-only memory (EEPROMs) or similar electronic device. Under a technique referred to as "backscatter modulation," the RF/ID transponders transmit stored data by reflecting varying amounts of an electromagnetic field provided by an RF/ID interrogator by modifying their antenna matching impedances. The RF/ID transponders can therefore operate independent of the frequency of the energizing field, and as a result, the interrogator may operate at multiple frequencies so as to avoid radio frequency (RF) interference, such as utilizing frequency hopping spread spectrum modulation techniques. The RF/ID transponders may either be passive, in which they extract their power from the electromagnetic field provided by the interrogator, or active, in which they include their own power source.
Since RF/ID transponders do not include a radio transceiver, they can be manufactured in very small, light weight and inexpensive units. Passive RF/ID transponders are particularly cost effective since they lack a power source. In view of these advantages, RF/ID transponders can be used in many types of applications in which it is desirable to track information regarding a moving or inaccessible object. One such application is to affix RF/ID transponders to work pieces moving along a conveyor belt of an assembly line. The RF/ID transponders would contain stored information regarding the particular assembly requirements for the work piece to enable automated equipment to operate on the work piece and perform certain tasks unique to the work piece requirements. This way, products having different assembly requirements can be sent down the same assembly line without having to modify the assembly line for each unique requirement. Another application for RF/ID systems is to collect information from a moving motor vehicle, such as for vehicle toll collection.
A drawback of conventional RF/ID systems is that it is difficult for the interrogator to accurately recover the information stored in the transponder when the transponder is moved relative to the interrogator. The signals received by the interrogator exhibit amplitude nulls and phase reversal when the round-trip transmission path delay (i.e., phase delay) between the interrogator and the transponder changes by more than one-quarter wavelength (80/4). With a carrier frequency of 2.4 GHz, 80 /4 would be equal to 3 centimeters (cm). It takes roughly 2 milliseconds (msec) to transmit a typical data packet at a rate of 40 kilobits per second (kpbs)). Thus, an RF/ID transponder moving at a rate of about 1,500 cm/sec, or 34 miles per hour (corresponding to 3 cm in 2 msec), relative to the interrogator, would likely exhibit undesirable amplitude nulls and phase reversals. Such a rate of speed would be expected in many RF/ID applications, and the amplitude nulls and phase reversals are further exacerbated at higher speeds. The amplitude nulls and phase reversals tend to mask actual data, making data recovery very difficult. As a result, RF/ID systems are not sufficiently reliable in recovering data under conditions in which the transponder is moving very quickly, such as the moving conveyor belt and motor vehicle applications discussed above.
Thus, it would be very desirable to provide an RF/ID interrogator having a capability of recovering data from a moving RF/ID transponder.